Optical encoders or tachometers are generally well known in the art. Basically, these devices comprise a stationary light source arranged to be mounted on a motor close to the motor shaft. A stationary receiving photo electric cell is provided in alignment with the light source and a rotating shutter secured to the shaft is caused to pass between the light source and photo cell to modulate the light beam to the photo cell. The frequency of output pulses corresponds to the R.P.M. of the motor. In these assemblies, it is vitally important that the light source and photo cell be properly aligned with respect to the rotating shutter in order that usable output pulses are properly generated. Moreover, it is important that sufficient light irradiate the photo cell to assure proper amplitude of pulses.
Accordingly, many presently available encoders provide for a permanent mounting of the light source in order that its position will be consistent in the mounting member portion of the assembly and thus consistent with the position of the photo cell. Further, a lens is usually provided for concentrating the light on the photo cell. However, where it is necessary to replace the light bulb or other light source, it cannot be assured that the light will be radiated in precisely the same direction towards the photo cell as was the case before. Primarily, the problem involved is that of the lamp filament itself which position may vary and thus vary the concentration of the light. Such concentration of the light is accentuated by the lens so that any misalignment of the lamp with the lens is exaggerated. Further, removal of the lens is necessary to replace the light bulb and in replacing the lens, consistency is not always assured so that the entire operation becomes somewhat of a burden if effective operation is to be assured after replacement of the light bulb.